1. Field of the Invention
The present invention relates to an exhaust gas purification device for an engine. More specifically, the present invention relates to a device which is capable of effectively removing a NO.sub.x component from exhaust gas.
2. Description of the Related Art
A three-way reducing and oxidizing catalyst is generally used as a means for removing pollutants from the exhaust gas of an internal combustion engine. The three-way reducing and oxidizing catalyst can remove three pollutants in the exhaust gas, i.e., CO, HC and NO.sub.x (nitrogen oxide) simultaneously when the air-fuel ratio of the exhaust gas is a stoichiometric air-fuel ratio.
However, it is known that the ability of the three-way reducing and oxidizing catalyst for removing NO.sub.x in the exhaust gas falls rapidly when the air-fuel ratio of the exhaust gas becomes higher than a stoichiometric air-fuel ratio (i.e., becomes leaner). Therefore, in engines operated in a lean air-fuel ratio such as lean burn gasoline engines or diesel engines, it is very difficult to remove NO.sub.x from the exhaust gas the use of three-way reducing and oxidizing catalysts.
To solve this problem, Japanese Unexamined Patent Publication (KOKAI) No. 62-106826 discloses a method for removing NO.sub.x components from exhaust gas of a diesel engine using a catalyst (or, NO.sub.x absorbent) which can absorb a NO.sub.x component in the exhaust gas in the presence of oxygen.
In the method disclosed by the above Publication, a vessel containing the catalyst (or, NO.sub.x absorbent) is disposed in the exhaust passage of a diesel engine, and the exhaust gas of the engine is introduced into the vessel during operation of the engine. Since the air-fuel ratio of the exhaust gas of the diesel engine is lean (i.e., concentration of oxygen component in the exhaust gas is high), the NO.sub.x absorbent absorbs the NO.sub.x component in the exhaust gas.
When the ability of the NO.sub.x absorbent decreases due to an increase in the amount of the absorbed NO.sub.x in the NO.sub.x absorbed, the exhaust gas flowing into the vessel is cut off, and a reducing agent such as hydrogen gas is introduced into the vessel. By cutting off the exhaust gas flowing into the vessel and introducing the reducing agent, the concentration of oxygen in the vessel becomes lower, and consequently, the absorbed NO.sub.x is released from the NO.sub.x absorbent and is reduced to nitrogen by reacting with the reduction agent. By releasing the absorbed NO.sub.x in the above process, the NO.sub.x absorbent recovers its original capacity for absorbing NO.sub.x in the exhaust gas. (Therefore, the process for causing the release of the absorbed nitrogen from the NO.sub.x absorbent and reduction thereof is called "a regenerating process of the NO.sub.x absorbent" in this specification.)
Since the air-fuel ratio of the exhaust gas of the diesel engine is lean, i.e., oxygen concentration of the exhaust gas is high, if the regeneration process is carried out in the presence of exhaust gas flowing into the vessel, a large amount of the reducing agent is required to consume the oxygen content of the exhaust gas flowing into the vessel to thereby generate an atmosphere of lower oxygen concentration in the vessel. Therefore, in the above method, a regenerating process is carried out by introducing the reducing agent while the exhaust gas flowing into the vessel is cut off.
By cutting off the exhaust gas flowing into the vessel, it is theoretically considered that, in the above method, the amount of the reducing agent required for the regenerating process can be reduced to a sum of the amount required to consume the oxygen content in the exhaust gas remaining in the vessel and the amount required for reducing the NO.sub.x released from the NO.sub.x absorbent.
However, the amount actually required for the regenerating process in the above method becomes much larger than the above theoretical value. This problem is explained with reference to FIGS. 1A and 1B. FIGS. 1A and 1B schematically illustrate the regenerating process disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 62-106826. In FIG. 1A, reference numeral 2 represents an exhaust passage of the diesel engine, and 3 represents a vessel containing a NO.sub.x absorbent 1 connected to the exhaust passage 2. Numeral 5 represents an exhaust shutter valve disposed in the exhaust passage 2 upstream of the vessel 3 to cut off the exhaust gas flowing into the vessel, and 4 represents a nozzle of the reducing agent supply device for supplying a reducing agent to the NO.sub.x absorbent 1 during the regenerating process.
As explained above, the exhaust shutter valve 5 is closed during the regenerating process in this method, and the reducing agent is supplied from the nozzle 4 under the condition in which no exhaust gas flow exists in the vessel 3. Because of the absence of the gas flow carrying the reducing agent, the reducing agent supplied from the nozzle 4 stays in the region near the nozzle 4 and forms a mass of a high concentration reduction agent. This reduction agent progressively diffuses in the vessel, and as time passes, a uniform mixture of the exhaust gas remains in the vessel and the reducing agent is formed. However, in the absence of the gas flow in the vessel, it takes a long time for the reducing agent to diffuse over the entire volume of the vessel. This causes an increase in the time required for the regenerating process to a level not practically acceptable.
Therefore, to reduce the time required for the regenerating process, it is necessary to continue to supply the reducing agent from the nozzle 4 even after the amount of the reducing agent theoretically required for the regeneration of the NO.sub.x absorbent has been supplied from the nozzle 4 so that the mass of the high concentration reducing agent gas replaces the exhaust gas in the vessel instead of diffusing therein. FIG. 1B schematically illustrates the change in the distribution of the concentration of the reducing agent within the vessel. The horizontal axis in FIG. 1B represents the distance from the nozzle 4 in FIG. 1A, and the curves (1) through (4) show the change in the distribution of the reducing agent as time passes. As seen from FIG. 1B curves (1) through (4), the reducing agent continuously supplied from the nozzle expels the exhaust gas from the vessel as time passes (curves (1) through (3)), and finally the vessel is filled with high concentration reducing agent gas.
This means that the amount of the reducing agent required in the above method is much larger than the amount actually required for regeneration of the NO.sub.x absorbent. This causes an increase in the running cost of the device due to higher consumption of the reducing agent, as well as problems such as that surplus reducing agent which is not consumed in the regenerating process being released into the atmosphere at the completion of the regenerating process, or by combining with NO.sub.x, forming other pollutants such as ammonia gas.
Also, the time required for the regenerating process becomes relatively long since a large amount of the reducing agent must be supplied from the nozzle to fill the entire volume of the vessel. Especially, if a liquid having a higher boiling point such as kerosene or gas oil is used as the reducing agent, the time required for the regenerating process becomes much longer since the evaporation of such liquid reducing agent takes a substantially longer time. Therefore, in the above method, it is practically difficult to use a liquid reducing agent having a higher boiling point.
Further, the activity of the NO.sub.x absorbent in releasing the absorbed NO.sub.x becomes more vigorous as the temperature of the NO.sub.x absorbent becomes high. Therefore, if the regenerating process is carried out at high temperature, the time required for completing the regenerating process can be reduced. When the reducing agent is supplied to the NO.sub.x absorbent, a part of the supplied NO.sub.x absorbent is oxidized by the NO.sub.x absorbent, and by this oxidation of the reducing agent, the NO.sub.x absorbent is heated. However, in the above method, a temperature rise in the NO.sub.x absorbent by the oxidation of the reducing agent is not sufficient due to the absence of oxygen since the atmosphere of the NO.sub.x absorbent is maintained in the high concentration reducing agent gas during the regenerating process.